The present invention relates to an apparatus for spatially resolved temperature measurement and to a method for spatially resolved temperature measurement.
Fiber-optic temperature measurement systems (Distributed Temperature Sensing—DTS) can use optical effects in optical fibers for spatially resolved temperature measurement. By way of example, the effect of Raman scattering can be used. This involves the radiation from a narrowband source of electromagnetic radiation (e.g. by way of a laser) being inelastically scattered in the fiber material. The ratio of the intensities of the scattered radiation with a shorter wavelength than the excitation (anti-Stokes scattered radiation) and the scattered radiation at a longer wavelength (Stokes scattered radiation) is temperature dependent and can be used for temperature determination. By using frequency (Optical Frequency-Domain Reflectometry—OFDR (EP0692705, EP0898151)) or pulse techniques (Optical Time-Domain Reflectometry—OTDR), the temperature along the fiber can be determined with spatial resolution. By way of example, such measuring systems can be used for monitoring fires in tunnels and channels, for monitoring power cables and pipelines and for mining oil and gas.
An apparatus and a method of the type cited at the outset are known from EP 0 692 705 A1, for example. In this case, a problem of the spatially resolved temperature measurement in optical fibers is the limited spatial resolution along the fiber.
In the case of pulse techniques, this is determined by the width of the laser pulses and the time resolution of the detection electronics. In the case of frequency techniques, the spatial resolution is limited by the maximum frequency. Known OTDR-DTS setups achieve spatial resolutions in the region of 1 m.
In the OFDR-DTS arrangements known to date, the optical output power of a semiconductor laser is modulated by modulating the laser current. The detection is effected by demodulating or mixing the electrical signals coming from the optical receiver. This may involve the use of homodyne detection (demodulation using the laser frequency) or else heterodyne detection (mixing with a frequency which is shifted in comparison with the laser). Heterodyne detection has the advantage that the downstream amplifiers can be operated in narrowband fashion on a fixed frequency.
Both the electrical laser modulation and the electrical demodulation are limited in terms of frequency.
The laser needs to be modulated using comparatively large currents (approximately 1A). The inductances in the supply lines and also the design of the laser allow the necessary modulation depths to be achieved only up to frequencies in the order of magnitude of 100 MHz.
For detecting the modulated light, photodiodes with transimpedance amplifiers are usually used. With the requisite DC coupling and the necessary gains, it is possible to implement frequencies in the region of 250 MHz.
Electrical modulation of the laser and electrical demodulation of the received signals can be used to achieve spatial resolutions of approximately 0.5 m.
An alternative to distributed temperature measurement in normal optical fibers is the use of Fiber Bragg Gratings (FBGs). Such FBGs can be introduced into optical fibers at short intervals and thus allow temperature measurements at high spatial resolution. However, the technique is very complex (each grating needs to be coded individually) and also allows only isolated measurements.
Numerous industrial applications and applications in the environment require distributed temperature measurements at spatial resolutions of 0.1 m or better. These spatial resolutions cannot be achieved with the known arrangements.